JPH05105955A - Hot rolling method for nonoriented silicon steel sheet having extremely superior magnetic property - Google Patents

Abstract

PURPOSE:To produce a nonoriented silicon steel sheet having extremely excellent magnetic properties by applying low temp. heating to a slab in order to prevent inclusions, such as MnS and AlN, from entering into solid solution again by using an atmosphere control type electric heating furnace. CONSTITUTION:At the time of hot-rolling a slab for nonoriented silicon steel having a composition consisting of <=0.0080% C, <=4% Si, 0.02-0.6% Mn, <=0.1% P, <=0.0050% S, <=1.0% Al, <=0.0030% N, and the balance essentially Fe, the slab is heated up to <=1150 deg.C by means of an atmosphere control type electric heating furnace provided to the position adjacent to a rougher, roughed directly after extraction from the furnace, and successively subjected to finish rolling. By this method, the nonoriented silicon steel sheet having extremely superior magnetic properties, such as iron loss, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of hot rolling with slab heating using an electric furnace of controlled atmosphere when producing a non-oriented electrical steel sheet having low iron loss and high magnetic flux density. It is about.

[0002]

2. Description of the Related Art Normally, when hot rolling a high grade non-oriented electrical steel sheet (JIS 50A470 or higher),
Slab heating is performed at 00 ° C to 1300 ° C, rough rolling is performed for 2 to 6 passes, and then rolled to form a hot rolled sheet. The operating conditions vary depending on the hot rolling mill used, but the slab thickness is 120 mm to 250 mm and the finishing rolling temperature is 750 ° C to 9 mm.
It is usual that the temperature is 50 ° C., the winding temperature is 500 ° C. to 700 ° C., and the hot rolled sheet thickness is 1.6 mm to 2.9 mm.

Then, cold rolling, annealing, etc. are performed to obtain a product, and the non-oriented electrical steel sheet exhibits magnetic properties by performing primary recrystallization in the final annealing step. At this time, it is known that primary recrystallization is less likely to occur or the growth thereof is suppressed as the amount of fine (several hundred to several thousand angstroms) sulfides or nitrides increases in the steel sheet. That is, the inclusions present in the slab form a solid solution by high temperature heating before hot rolling, and finely precipitate in the temperature range of 1100 to 900 ° C. during the subsequent hot rolling. For this reason, it is necessary to perform a high-purification treatment that removes S, N, etc. quite thoroughly at the steelmaking stage. However, this treatment greatly increases the manufacturing cost and reaches a level at which these can be completely removed. I can't cut it.

In order to solve such a problem, there is a method of lowering the slab heating temperature to prevent the inclusion of the inclusions present in the steel from forming a solid solution, for example, in Japanese Patent Laid-Open No. 60-190521.
It is disclosed in the publication. That is, in this publication, when the slab is heated, it is heated so as not to exceed 1150 ° C.
Although it is described that the re-dissolution of precipitates such as nS and AlN is suppressed, in the gas heating method for heating the slab normally, the temperature drop of the width direction edge part of the material to be rolled during hot rolling is reduced. Therefore, there is a restriction that lowering the heating temperature causes difficulty in rolling due to an increase in rolling load.

[0005]

As described above, in order to suppress re-dissolution of MnS or AlN which adversely affects magnetism in the heating furnace, it is better to lower the heating temperature of the slab and the aging heat temperature. It is preferable to set the temperature to 900 ° C. or lower which can prevent re-dissolution. That is, if the solid solution can be completely prevented, there will be no fine precipitation during hot rolling.
And AlN can be made harmless. From this point of view, the present invention prevents low temperature heating of the slab that can suppress re-dissolution of the inclusions described above and prevents temperature decrease in the width direction edge portion during hot rolling, and thereby has extremely excellent magnetic properties. It is an object of the present invention to provide a hot rolling method for non-oriented electrical steel sheets.

[0006]

In order to achieve the above object, the present invention has the following structures. That is, (1) [C]
0.0080% or less, [Si] 4% or less, [Mn] 0.
02-0.6% or less, [P] 0.1% or less, [S] 0.
0050% or less, [Al] 1.0% or less, [N] 0.0
In hot-rolling a slab for non-oriented electrical steel consisting of 030% or less and the balance substantially Fe, the slab is heated to 1150 ° C. or less in an atmosphere-controlled electric heating furnace provided adjacent to a rough rolling machine, Rough rolling immediately after extracting the slab from the furnace, is a hot rolling method of non-oriented electrical steel sheet having extremely excellent magnetic properties, characterized by performing subsequent finish rolling,
(2) The atmosphere-controlled electric heating furnace is provided with a multi-stage coil capable of controlling the output in the vertical direction, thereby heating the slab for non-oriented electrical steel to 1150 ° C. or less, which is extremely excellent. It is a method of hot rolling a non-oriented electrical steel sheet having excellent magnetic properties.

In the above invention, the atmosphere-controlled electric heating furnace to be used is provided with a plurality of induction heating coils in multiple stages in the vertical direction, and the output of each induction heating coil is adjusted to adjust the edge portion of the charging slab width direction. It is preferable to be configured so that it can be heated to a temperature. The present invention will be specifically described below. In the method for manufacturing a non-oriented electrical steel sheet, the slab is usually heated to a high temperature prior to hot rolling as described above, but at this time, the precipitates such as MnS and AlN in the steel are dissolved, and Fine reprecipitation occurs in the so-called hot rolling process. The formation of these fine precipitates prevents the recrystallized grains from sufficiently growing during the finish annealing of the steel strip in the subsequent step, resulting in deterioration of magnetic properties.

On the other hand, even if heating is carried out at a relatively low temperature in order to prevent such a defect of high temperature heating, when this is carried out in a gas type heating furnace, it is baked from the outside of the slab by a direct flame method. Therefore, it takes a long time to heat up to the central portion, that is, to heat the heat. Also, if baking is attempted in a short time, the vicinity of the slab surface must be overheated. Also, it is not possible to bake each slab,
The slab that has been baked almost uniformly has a large decrease in the widthwise edge portion during rolling. Therefore, for example, this edge temperature is the lower limit temperature at which rolling is possible (normal rolling finish outlet temperature is approximately 7
The lower temperature limit for firing the slab is empirically set from the point of (00 ° C.), but the lower limit temperature cannot be set low. In addition, since the gas heating furnace usually has a considerable distance to the rough rolling mill due to equipment problems, the slab also has a large temperature drop during the transportation from the heating furnace to the rough rolling mill after the extraction, which results in the heating of the slab. This is a major factor in raising the furnace extraction temperature. As shown in FIG. 1, the heating furnace extraction temperature is about 1000 ° C. when the rolling finish outlet temperature capable of hot rolling by heating in a gas heating furnace is 700 ° C., and the conventional heating temperature is in such a low temperature range. Expansion is not acceptable.

Therefore, one of the features of the present invention is that the slab heating before hot rolling is performed in a low temperature range of 1150 ° C. or less and in a short time. This low-temperature short-time heating can be achieved by using an electric heating furnace capable of controlling the inside of the furnace to a non-oxidizing atmosphere. In this electric heating furnace, it is preferable that a plurality of induction heating coils are vertically arranged in multiple stages, and the output of each induction heating coil can be adjusted. That is, even if induction heating is performed, if the entire length and width of the slab are uniformly baked, the temperature drop at the slab edge during rolling is inevitably larger than at the center in the width direction.
As a result, the slab edge lowering temperature must be set from the point that the slab edge portion reaches the lower limit temperature at which rolling is possible. Therefore, according to the method of the present invention, it is possible to further lower the firing lower limit temperature of the slab by increasing the output of the coil at the edge portion, which has a low temperature in the rolling stage, and heating the coil above the central portion.

Such induction-heated slab is subjected to rough rolling immediately after extraction from the furnace. The temperature drop of the slab extracted from the furnace should be prevented as much as possible, especially at the edge, because the temperature is likely to drop, so that the electric power should be placed close to the rough rolling mill, that is, as close as possible to the equipment or due to the operation. Install a heating furnace to shorten the transfer time to the rough rolling mill. The electric heating furnace is preferably set in a non-oxidizing atmosphere, for example, an Ar or N gas atmosphere, because this can prevent the generation of scale on the slab surface.

In the present invention, the heating temperature is limited to 1150 ° C. or lower because MnS and A are heated by heating at a temperature below this temperature in an atmosphere-controlled electric heating furnace for a short time.
This is for suppressing the re-dissolution of inclusions such as 1N and the like, and the lower limit thereof is within the range where hot rolling is possible, and is not particularly limited. It is preferable to heat at the lowest possible temperature in this range.

Next, the reason why the component composition is limited to the above range in the present invention will be explained. When [C] is contained in a large amount, the austenite-ferrite two-phase range is widened during hot rolling, and it takes a long time for decarburization annealing and is disadvantageous in terms of magnetic properties. 008%
Below. [Si] is usually set to Si ≦ 4% contained in the non-oriented electrical steel sheet. In order to obtain a low iron loss, it is better to increase Si in order to increase the specific resistance, but if it exceeds 4%, the cold ductility deteriorates, so it is preferable to limit it to 4% or less. [Mn] is contained in an amount of 0.02 to 0.5%.
In this, Mn / S was set to 15 or more, and the lower limit was made 0.02% in order to prevent red heat embrittlement. On the other hand, the upper limit of 0.5%
If it exceeds, solid solution hardening is caused and punching workability is deteriorated.

Next, in the present invention, regarding [S] and [N] that constitute the cleanliness, [S] forms fine sulfides or oxysulfides and is harmful to increase the primary recrystallization temperature. In order to exert the effect, it is desirable that the amount is as small as possible. However, if the low temperature heating shown in the present invention is carried out, the effect of preventing the deterioration of magnetism due to these precipitates can be confirmed. Therefore, the upper limit of [S] is 0.005 considering the non-oriented electrical steel sheet.
0%, [N] is preferably as small as possible in order to minimize AlN precipitation during hot rolling and weaken the inhibitory effect of primary recrystallized grains as in [S].
It was set to 0%.

[P] has the effect of increasing the hardness of the steel sheet and improving the punching property, but on the other hand, if its content increases, iron loss and magnetic flux density deteriorate, so it is made 0.1% or less.
From the viewpoint of magnetic properties, the higher the content of [Al], the more preferable it is, but if the content exceeds 1%, the cold ductility deteriorates, so the upper limit was made 1%. There is no particular limitation on other elemental elements, but for the purpose of further improving the magnetic flux density by adding [Ca] in order to stabilize the trace amount [S] remaining in iron, [ It is also effective to add Sn] or [Cu].

[0015]

Example 1 [C] 0.0020%, [Si] 3.0
%, [Mn] 0.2%, [S] 0.0010%, [A
1] 0.6%, [TN] 0.0015% containing slab for non-oriented electrical steel sheet is charged into an electric heating furnace (induction heating furnace) installed close to a rough rolling mill, and the inside of the furnace is charged. It was heated to 900 ° C. in a nitrogen gas atmosphere. Immediately after the slab was extracted from the furnace, the slab was bitten into a rough rolling machine to perform rough-finish rolling to 2.0.
mm hot rolled steel sheet. Finishing outlet temperature at this time is 730 ℃
Met. This steel plate is annealed at 900 ° C for 30 seconds, hot-rolled after pickling, and cold-rolled at a reduction rate of 75% to 880 ° C.
Finish annealing was performed for 30 seconds. Iron loss of the obtained steel sheet (W1
5/50), a very good value of 2.00 w / kg was obtained. On the other hand, when slabs with the same composition were heated to 900 ° C in a direct-fire gas heating furnace and then hot-rolled, the finish rolling inlet temperature dropped to 660 ° C, and the hot-rolled sheet thickness gauge could not be obtained. It did not become a product.

[0016]

Example 2 [C] 0.0030%, [Si] 1.0
%, [Mn] 0.2%, [S] 0.0030%, [A
l] tr. , [TN] 0.0015% containing slab for non-oriented electrical steel sheet is charged into an electric heating furnace (induction heating furnace) installed in the vicinity of a rough rolling machine, and the inside of the furnace is made to have a nitrogen gas atmosphere. Heated to ° C. Immediately after the slab was extracted from the furnace, the slab was bitten into a rough rolling machine and subjected to rough-finish rolling to obtain 2.3.
mm hot rolled steel sheet. After pickling this steel plate, the reduction rate is 78%
Cold-rolled, and finish annealing was performed at 800 ° C. for 30 seconds.

On the other hand, as a comparative method, a slab having the same composition was heated to 1000 ° C. in a direct-fired gas heating furnace and then hot-rolled. As described below, the temperature drop during hot rolling was large, and the finish outlet temperature was as low as 700 ° C. This hot-rolled steel sheet was pickled in the same manner as above, cold-rolled at a reduction rate of 78%, and finish-annealed at 800 ° C. for 30 seconds. .

FIG. 1 shows the heat from the furnace extraction to the winding of the slab heated in the induction heating furnace adjacent to the rough rolling mill of the present invention and the slab heated in the gas heating furnace of the comparative example in this embodiment. The temperature transition during hot rolling is shown. In all cases, a rolling outlet temperature of 700 ° C., which is within the rolling range, could be secured, but in the comparative example (direct heating type gas heating), the slab heating furnace extraction temperature had to be 100 ° C. higher than that of the present invention. As is clear from the figure, in the comparative example, the distance between the rough rolling mills is large, and a temperature drop of about 80 ° C. is already observed before the rough rolling, whereas in the inventive example, the moving distance (time) is increased. This is because rough rolling can be started with a low temperature drop of 10 ° C. Further, in the rough rolling, in the comparative example, 160
In the present invention example, the temperature drop is 130 ° C., and similarly in the finish rolling, the temperature drop of the present invention example is small. That is, in direct-fire heating, since heating is from the surface of the slab, it is difficult to bake to the center of the slab thickness, and as the rolling progresses, the temperature of the surface and the central portion averages, so the temperature drop is large. On the other hand, in the case of induction heating,
On the contrary, since the slab is baked from the central portion in the thickness direction, the temperature difference between the slab surface and the central portion is small, and the temperature drop during rolling is remarkably smaller than that of the direct flame type.

Table 1 shows the results of measuring the magnetic flux density (B50) and iron loss (W15 / 50) of each of the obtained steel sheets.

[0020]

[Table 1]

As is apparent from the table, the method of the present invention by electric heating (induction heating) has excellent characteristics as compared with the comparative example by direct gas heating.

[0022]

As described above, according to the present invention, the slab is heated at a low temperature by the controlled atmosphere induction heating and hot-rolled to prevent the reprecipitation of sulfides and nitrides and render them harmless. It is possible to manufacture a non-oriented raw electrical steel sheet having high magnetic properties.

[Brief description of drawings]

FIG. 1 is a diagram showing a slab surface (tail) temperature history in a time transition during hot rolling.

Claims (2)

[Claims] 1. [C] 0.0080% or less, [Si] 4
% Or less, [Mn] 0.02 to 0.6% or less, [P] 0.
1% or less, [S] 0.0050% or less, [Al] 1.0
% Or less, [N] 0.0030% or less, balance substantially Fe
At the time of hot rolling the slab for non-oriented electrical steels, the slab is heated to 1150 ° C. or less in an atmosphere-controlled electric heating furnace provided adjacent to the rough rolling mill, and the slab is subjected to rough rolling immediately after extraction from the furnace. A hot rolling method for a non-oriented electrical steel sheet having extremely excellent magnetic properties, which is characterized by carrying out finish rolling. 2. The slab for non-oriented electrical steel is heated to 1150 ° C. or less in an atmosphere-controlled electric heating furnace provided with a multi-stage coil capable of controlling output in the vertical direction. Hot rolling method for non-oriented electrical steel sheet having extremely excellent magnetic properties.